Vein wall structure and properties will be quantitatively studied and related to the ability of the venous system to alter blood volume distribution and central venous pressure to meet environmental stresses. Wall structure will be determined using quantitative computer assisted image analysis and histologic stains specific for smooth muscle, collagen, and elastin. The procedure will allow content, distribution, and orientation of the wall components to be measured. The mechanical properties of the vein segments will be determined in vitro using photographic and/or TV images and in situ using radiographic procedures. Anatomical variations in canine veins will be used to examine the relationship of structure and function, particularly constrictive ability. The rate and amount of smooth muscle proliferation in response to increased venous pressure will be determined as will the effects of changes in axial strain on segment volume. The ability of smooth muscle to augment venous return by increasing the time constant for creep will also be examined. In situ experiments will be performed to relate observed changes in structure and properties to compensations during hemorrhage and carotid sinus hypotension.